French researchers make an astonishing discovery about communication during sleep – Futura

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Sleep is not a state that completely isolates us from our environment: we are able to hear and understand words while we sleep. These observations, the result of a close collaboration between researchers from Inserm, the CNRS, the Sorbonne University and the AP-HP at the Brain Institute and the Sleep Pathology Department of the Pitié-Hospital Salpêtrière AP-HP in Paris, represent the very definition of in question sleep and the clinical criteria that make it possible to distinguish between its different stages. They are detailed in a new study published in the journal Nature Neuroscience.

Sleep is generally defined as a period of time when the body and mind rest as if separated from the world. However, a new study led by Delphine Oudiette, researcher at Inserm, Isabelle Arnulf (Sorbonne University, AP-HP) and Lionel Naccache (Sorbonne University, AP-HP) at the Brain Institute shows that the boundary between wakefulness and sleep is very large is more porous than it seems.

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Scientists have shown that sleepers, without any special disorders, are able to perceive verbal information conveyed by a human voice and respond to it with contractions of facial muscles. However, this amazing ability manifests itself at times in almost all phases of sleep – as if windows of connection to the outside world were temporarily opened on this occasion.

These new data on sleep behavior suggest that it may eventually be possible to develop standardized communication protocols with sleeping people to better understand how mental activity changes during sleep.

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A thousand and one variations of consciousness

“Although it sounds familiar because we enjoy it every night, sleep is a very complex phenomenon. Our research has taught us that waking and sleeping are not stable states: they are both a mosaic of conscious moments… and moments that appear not to be,” explains Professor Lionel Naccache, neurologist at the AP-HP Pitié-Salpêtrière Hospital and researchers in neuroscience.

It is important to better understand the brain mechanisms underlying these intermediate states between wakefulness and sleep. “Unregulated, they can be accompanied by disorders such as sleepwalking, sleep paralysis, hallucinations, the feeling of not sleeping at night or, on the contrary, sleeping with your eyes open,” explains Professor Isabelle Arnulf, head of the sleep pathologies department at the AP-HP Pitié-Salpêtrière hospital .

However, to distinguish the state of wakefulness and the different stages of sleep, we have so far used simple and imprecise physiological indicators, such as specific brain waves that are visualized thanks to electroencephalography. These indicators do not allow us to understand in detail what is going on in the minds of sleepers, especially since they sometimes contradict what they say. “We need finer physiological measurements that are attuned to the sleeper’s feelings and ability to respond to the outside world; This serves to better define one’s level of alertness,” adds Delphine Oudiette, Inserm researcher in cognitive neuroscience.

A game between unconsciousness and clarity

The research team* therefore took this approach and recruited 22 people without sleep disorders and 27 narcoleptics – i.e. victims of unruly sleep episodes.

Narcoleptics have the peculiarity that they have many lucid dreams, i.e. dreams in which they are aware of sleeping and whose scenario they can sometimes create themselves. In addition, they easily and quickly reach paradoxical sleep (the stage at which lucid dreams occur) during the day, making them good candidates for studying consciousness during sleep under experimental conditions.

“One of our previous studies showed that in lucid paradoxical sleep, two-way communication from the experimenter to the dreamer and vice versa is possible,” explains Delphine Oudiette. Now we wanted to know whether these results could be extrapolated to other sleep phases and to people who don’t have lucid dreams.”

Study participants were asked to take a nap. The researchers gave them what they called a “lexical decision test,” in which a human voice spoke a series of real words and made-up words. To place them in one or other of these categories, participants had to respond with a smile or a frown. Throughout the experiment, participants were monitored using polysomnography – a comprehensive examination that recorded their brain and heart activity, eye movements and muscle tone. Finally, upon waking, participants were asked to report whether they had had a lucid dream during their nap and whether they remembered interacting with anyone.

“Most participants, whether narcoleptics or not, were able to respond correctly to verbal stimuli while sleeping. These events certainly occurred more frequently during lucid dream episodes characterized by high levels of consciousness; “But we occasionally observed them in both groups, in all sleep phases,” explains Professor Isabelle Arnulf.

“Most participants, whether narcoleptics or not, were able to respond correctly to verbal stimuli while sleeping.”

Connection windows that are announced by an acceleration of brain activity

By combining this physiological and behavioral data and participants’ subjective reports, the researchers also show that it is possible to predict the opening of these windows of connection with the environment, i.e. the moments when sleepers were able to respond to stimuli. These were announced by an acceleration of brain activity and by physiological indicators normally associated with rich cognitive activity.

“In people who had a lucid dream during their nap, the ability to communicate with the experimenter and recount this experience upon waking was also characterized by a specific electrophysiological signature,” adds Professor Lionel Naccache. Our data suggests that lucid dreamers have privileged access to their inner world and that this heightened awareness extends to the outside world as well.”

Further research is needed to determine whether the multiplication of these windows is related to sleep quality and whether they could be used to improve certain sleep disorders or promote learning. “More advanced neuroimaging techniques such as magnetoencephalography and intracranial recording of brain activity will help us better understand the brain mechanisms that control the behavior of sleepers,” concludes Delphine Oudiette.

Ultimately, this new data could help revise the definition of sleep, a state that is ultimately very active, perhaps more conscious than we imagined, and open to the world and others.

This study was funded by the National Research Agency and the French Society for Sleep Research and Medicine.

* The participation of graduate students Başak Türker, Esteban Munoz Musat and Emma Chabania was crucial to the completion of this work.